It is well known in the art that MSZ compositions can be obtained by mixing zirconia or hafnia with appropriate amounts of other metal oxides known as the stabilizing oxides. The most used stabilizing oxides are rare earth and/or alkaline earth oxides, especially yttria and magnesia. The needed intimate mixing between the zirconia and the stabilizing oxide, along with the needed type and size of crystallites in the product, may be obtained by precipitating from solution, as exemplified by U.S. Pat. No. 4,501,818 of Feb. 26, 1985 to Rossi, or by rapid solidification from a melt, as described in U.S. Pat. No. 4,565,792 of Jan. 21, 1986 to Knapp, the entire specification of which is hereby incorporated herein by reference.
The rapid solidification process produces an initial product that consists primarily of hollow small spheres from 30 to 6500 microns in diameter, with a median diameter of about 2000 microns. This initial product is denoted hereinafter as "rapidly solidified crude" or "RS crude".
The preferred method of final comminution of the MSZ produced by rapid solidification is wet vibratory milling, such as that as accomplished by a commercial Sweco mill. This milling step produces a final average particle size of less than 2 microns. The vibratory mill, however, can not effectively be used directly on much of the RS crude, because the mill operates efficiently only on particles about eight hundred microns or less in size. An intermediate comminution step is thus needed before vibratory milling.
As taught in U.S. Pat. No. 4,565,792, this intermediate step was performed by conventional mechanical crushing. While this method was adequate, it sometimes was found to introduce undesirable levels of metallic and other impurities that would reduce the toughness of final objects made from the MSZ powders subsequently produced. An object of this invention is to provide an improved method for this intermediate comminution. The method also can be useful for recycling rejected or broken sintered objects of MSZ, thereby recovering the fairly expensive zirconia content without requiring the energy expenditure for remelting.
Beginning in 1981, reports have appeared in the non-patent literature that sintered bodies made from MSZ's were susceptible to substantial reductions in toughness, and sometimes even to loss of integrity, when aged at temperatures above 100 C. in water or between about 150 and 400 C. in air. These reports are summarized, and the latest work of this type known to the applicants is reported, in M. Yoshimura et al., "Role of H.sub.2 O on the degradation process of Y-TZP", 6 Journal of Materials Science Letters 465-67 (1987). ("Y-TZP" in this title indicates that yttria is the stabilizing oxide.)